Process of electrodepositing chromium



50 the character of Patented Mar. 10, 1931 TES PATENT OFFICE WILLIAM S.EATON, OF SAG HARBOR, NEW YORK PROCESS OF ELECTRODEPOSITING CHBOMIUM noDrawing.

My invention relates to a process of electrodepositing chromium, andmore particularly to a process involving theuse of a chromic acidsolution.

In the electrolytic depositing of chromium, particularly in the platingof various articles, great difficulties have been experienced, due inpart to the varying resistance conditions in the bath during the platingoperation, and

in part to difficulties in securing a continuous, uninterrupted depositof chromium upon the surface of the article. There have also beendificulties in securing a conditionin the bath whichv will securereliability in the electrolytic action.

i In most baths heretofore used there is a strong ebullition of noxiousfumes making the depositing operation somewhat hazardous to workers.

In some instances it is found that while an apparently satisfactorychromium surface is secured, after a shortinterval the chromium platingwill strip or fleck from portions of the surface due to corrosion of thematerial upon which the chromium is deposited beneath the platedsurface. 7

Great difliculties have also been experienced in securing a uniformdeposition of the chromium throughout the entire surface With the aboveconditions in mind, I have developed a process for theelectro-deposition of chromium in which the ingredients of the bath areof a character which will not only avoid the presence of fumes injuriousto the workmen, but which will offer a low, substan- 't ially uniformresistance to the flow of the electric current, thus avoiding thosedimculties arising from a. failure, during the procem, to continuouslydeposit the chromium at a uniform rate due to the development ofexcessive resistance throughout, or in different parts of the bath,whatever the cause of such phenomena may be. a

A bath used in the process of my invention is ready for immediate useupon the mixing of the solution, a u the process may be carried on atordinary f ory temperatures or at a slightly elevated temperature,according to the surface desired, or the "Application filed October 281926. Serial No. 144,679.

rapidity with which it is desired to throw the chromium. h

' In 'electro-depositing chromium, the reactions are accompanied byadecompositi'on of the water of the electrolyte, and the presence ofminute bubbles. of hydrogen at the cathode, which interfere with theadherence of the reduced chromium salt. There is also, with some baths,a formation of a brownish film upon the cathode which must be removedtherefrom at the completion of the plating process.

In the bath used with-the process of my invention, the cathode is keptfree of hydrogen gas bubbles, and the deposited chromium, when thecathode is removed from the bath, is in a clean, bright conditionwithout any evidence of the brown scum above referred to, thus requiringa mere washing of the cathode to-clean it.

I-find it essential to thoroughly clean the surface of the cathodebefore immersing it in the bath, and to protect the surface thereof fromthe action of oxygen thereon with a film gf (gear water, before soimmersing it'in the at v In the practice of the process of myinvention,I form a bath consisting of a solution, the active ingredients of whichconsist solely of chromic acid (CrO and sodium carbonate (Na CO theseingredients being added to the bath in the form of a saturated solutionof the chromic' acid, and a solution of the sodium carbonate of adensity of about 16 Baum.

I have gotten highly satisfactory results by using to each gallon ofwater in the solution, 20 ounces'of .the chromic acid solution and 10ounces of the sodium carbonate solution. These proportions, however, aresub-' ject to variation within a reasonable range,

without materially impairing the efiiciency of the bath. a

While it is possible to use either a lead or a chromium anode, I havefound that the latter gives the best results, since with it the ehromiumcontent of the bath is replenished to an extent to keep the bath in goodworking condition. By raising the temperature of the bath to from to iF., a bright, smooth bath to from 110 adhering deposit upon the cathodeof metallic chromium may be immediately secured with the use of anelectric current at a voltage of from 5 to 10, and an amperage of from 1to 2, or more, per square inch of the cathode.

The use of sodium carbonate (soda ash). not only reduces the resistanceof the electrolyte, but during the reactions adjacent the cathode,apparently forms combinations .with the hydrogen causing the sodiumcarbonateto recombine with the water of the bath before it has beenreduced to its metallic form and precipitated. The sodium carbonate alsoapparently has the effect of keeping the cathode clean in respects otherthan avoiding the accumulation of hydrogen bubbles upon and adjacent thecathode. It also appears to prevent the colloidal condition of thesolution at the cathode, since there is an absence of the brownishcolloidal film upon the surface of the cathode when removed from thebath.

In a bath containing sodium carbonate and chromic acid without otheringredients than water, the only metallic salts which may be reduced andthrown upon the cathode, are those contained in the chromic acid, andhence the deposited surface will consist of pure chromium applieddirectly to the metal of the cathode.

By using the higher temperatures in the bath as stated above, thehardness of the de posit and the brilliancy of its finish are increased.With a lower temperature bath, the surface will be somewhat duller andwill also be somewhat softer. Under any circum stances, however, thedeposited chromium will be very hard, and when a bright finish issecured in the bath, no subsequent polishing or bufling is required.Ordinarily in plating with chromium, a deposit of from .0001 to .0002 ofan inch is all that is required.

With the process of my invention, a flash of chromium may be applied tothe cathode instantly upon placing it within the electrolytic bath, andwhile the time interval may vary :with different temperatures in thebath, very excellent results can be secured by intervals of from twentyminutes to one hour. Of course if it is desired to increase thethickness of the deposit, this time interval may be increased, therebeing no tendency of polarization of the bath, or an increase of itsresistance, with a prolongation of the time interval.

If it be desired to remove the chromium surface from the cathode, thismay be done by using an alkaline bath in which the cathode is immersed,raising the temperature of this to 130 F., and reversing thedirection'of flow of the current, using the article from which thechromium surface is to be stripped as the anode.

The thorough cleaning of the cathode before its immersion in the bath isnecessary to ure to the control of the production of a chromium surfacethereon of the desired fine texture and continuity. In cleaning thecathode, I immerse same in an alkaline bath, using it as a cathode of anelectrolytic couple, the anode of which may be carbon plates or graphiterods. The voltage and amperage used is the same as that used in theplating bath. A very short time is required to thoroughly cleanse thecathode, three minues having been found ample. The cathode is thenremoved from the cleaning bath, thoroughly rinsed in clear water, and

immersed in the plating bath, the handling of the plate being donewithout disconnecting it from the electrical source.

The process of my invention may be used in surfacing flat or irregularlyshaped articles, chromium plating being particularly desirable as asurfacing for articles which require frequent cleaning or are subject tocorrosion as a result of exposure. The hard ness of the chromium permitsit to be cleaned with the use of various detergents and abrasives,since'such will have no effect whatever upon the surface except theremoval of superficial matter therefrom. The cleaning action need not beaccompanied by any polishing, since the removal of superficial depositsthereon will merely expose the finish of the surface, which will notbecome dim or dull by oxidation, corrosion or otherwise.

Chromium surfacing is particularly desirable in plumbing fittings, infittings for boats, ships and automobiles, in master plates used inengraving machines, in copper and steel printing plates, and in variousother articles wherein resistance to wear, or the avoidance of corrosiveaction of the elements thereon are matters of importance. A chromiumsurface while very attractive in appearance by reason of its blue-grayand other color effects, nevertheless possesses advantages apart fromits pleasing appearance. I

Notwithstanding the hardness of the chromilim surface. the article maybe readily formed after the chromium surface has been applied thereto,without the formation of minute cracksor checks. The surfacing in theevent of long continued wear may be readily removed and restored withoutinjury to the surface of the cathode or article upon which the chromiumis electro-deposited.

I attribute the successful results secured by the process of myinvention in large measthe hydrogen upon and adjacent the cathode, whichof itself will lower the resistance of the bath, although the presenceof sodium carbonate in the bath will contribute largely toward anincreased conductivity of the bath and uniformity in the resistancecondition, irrespective of the age of the bath or its extent of use.

What effect the sodium carbonate has in reducing the chromic acid, Ihave been unable to determine. Its chief utility appears to be thereduction of the resistance of the electrolyte, the reduction of surfacetension at the cathode resulting from either the rapid separation ofhydrogen bubbles from the cathode or the combination of hydrogen withcarbonate in a man- 7 the oxygen of the sodium ner to avoid the presenceof free hydrogen upon or adjacent the cathode and preventingconstituents the depositing of the colloids upon the cathode.

The solution is not only simple as the nature of the ingredientsentering therelnto, but requires no acquired skill in the forming of thebath or in its upkeep.

While certain gases are evolved during the electrolytic action, thevolume of such is small, and the eifect of same upon the worker is notat all serious, even if the precaution is not taken to provide a hoodand vent, or other methods for dissipating these gases. 1

It is not my intention to limit myself to the precise temperatures, timeintervals, or the of the bath as herein described,

it being apparent that such may be subjected to varlation withoutdeparting from the spirit and scope of the invention.

Having described the invention, what I claim as new and desire to haveprotected by Letters Patent, is

1. The herein described process of electrodepositing chromium,consisting in cleaning the surface'of the article upon which thechromium is to be deposited, immersing the article in an electrolyticbath, the active ingredients of which consist solely of chromic acid andsodium carbonate in solution in water, and in operative relation to ananode, and passing an electric current through a circuit containingtherein the anode, the electrolyte and the article as the cathode.

2. The herein described process of electrodepositing chromium,consisting in cleaning the surface of the article upon which thechromium is to be deposited, immersing the article in an electrolyticbath, the active ingredients of which consist solely of chromic and andsodium carbonate insolution in water, each gallon of water in the bathcontainingsubstantially 20 ounces of a saturated solution of chromicacid, and 10 ounces of a solution of sodium carbonate having a densityof 16 Baum, and in operative relation to an anode, and passing anelectric current through a circuit containing therein the anode, theelectrolyte and the article as the cathode.

In witness whereof I have hereunto afiixed my signature this 20th day ofOctober, 1926.

. WILLIAM S. EATON.

